<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    #app{
        width: 100px;
        height: 100px;
    }

</style>
<body>
    <div id="app">



    </div>

</body>
<script src="https://cdn.bootcdn.net/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<script>
    $(function () {
        console.log($('#app'));
        $('#app').css('background','red')

    })
</script>

</html>